Re: [TLS] A small detail in HMAC key generation for Finished message

[Hubert Kario <[e-mail]>]

On Monday 04 January 2016 09:44:57 Eric Rescorla wrote:
> On Mon, Jan 4, 2016 at 9:22 AM, Hubert Kario <[e-mail]> 
wrote:
> > On Thursday 24 December 2015 01:04:59 Christian Huitema wrote:
> > > On Wednesday, December 23, 2015 3:05 PM, Eric Rescorla wrote:
> > > >> Similarly, in the HKDF-Expand-Label, do we assume a final null
> > > >> byte
> > > >> for the "label"?>
> > > > 
> > > > No. I wonder if we should instead add the '\0' explicitly in the
> > > > 4.8.1 for maximal clarity.
> > > 
> > > Either that, or just remove the trailing 00 from the binary
> > > description.
> > 
> > the 0-byte is a C-ism that looks like a wart to me
> > 
> > neither of the previous TLS versions used null-terminated C-style
> > strings so why TLS1.3 should? Especially in just one place
> 
> The idea is to make this prefix-free. I added it as an explicit byte
> but would
> be ok with a different separator as long as we banned it from the
> context strings.

Calling it explicitly a separator would be less confusing.

Advising implementers to check other values passed in for it and 
aborting if detected would be even better
-- 
Regards,
Hubert Kario
Senior Quality Engineer, QE BaseOS Security team
Web: www.cz.redhat.com
Red Hat Czech s.r.o., Purkyňova 99/71, 612 45, Brno, Czech Republic